A class D audio amplifier is a well-known type of audio power amplifier which is generally recognized to provide energy efficient audio drive of loudspeakers by switching a pulse width modulated (PWM) or pulse density modulated (PDM) signal across the loudspeaker load. Class D audio amplifiers typically comprise an H-bridge based output stage coupled to the loudspeaker load to apply an oppositely phased pulse width modulated audio signals across the loudspeaker. An inductor-capacitor (LC) lowpass filter is normally inserted between the H-bridge based output stage and the loudspeaker load to suppress carrier wave components in the PWM or PDM output signal. Several modulation schemes for pulse width modulated audio signals have been utilized in prior art PWM based class D amplifiers. In so-called AD modulation, the pulse width modulated audio signal at each output terminal or node of the H-bridge is switched, or toggles, between two different levels in opposite phase. The two different levels typically correspond to the upper and lower power supply rails, respectively, such as the positive and negative DC supply rails of the class D amplifier. In so-called BD modulation, the pulse width modulated signal across the loudspeaker load is alternatingly switched between three levels of which two levels correspond to the above-mentioned upper and lower DC power supply rails and the third level is a zero level that is obtained by simultaneously pulling both sides of the loudspeaker load to one of the DC power supply rails. Both of these modulation schemes generate rather large ripple current in the output inductor of the LC lowpass filter when the class D amplifier is idling which cause significant power losses. This disadvantage has typically been tolerated and controlled to a certain extent by use of relatively large inductors of the LC lowpass filter. However, such large inductors lead to significant increase of costs and size of the class D amplification solution or assembly.
So-called multilevel PWM modulation is a particular advantageous form of pulse width modulation of audio and other signals and possesses numerous benefits over traditional AD and BD modulation as described in detail in the applicant's PCT application No. WO 2012/055968. This kind of multilevel PWM modulation typically involves one or several so-called flying capacitors coupled to the output stage to store a self or internally generated half supply voltage level on an external capacitor. This half supply voltage level is the source of the third voltage level of the PWM output signal. Balancing of the one or more flying capacitors is linked to the accuracy of the multiphase PWM signals. Therefore, multilevel PWM modulation requires accurate multiphase PWM signals to achieve optimum performance. These multiphase PWM signals are preferably generated by a multiphase pulse width modulator which derives the multiphase PWM signals from respective high precision multiphase analog triangle waveforms. The high precision multiphase analog triangle waveforms are used to define the sampling, via respective comparator circuits, of an output signal of an analog loop filter of the multilevel Class D amplifier.
However, generating such accurate multiphase PWM signals presents a significant challenge for various reasons, in particular generating sufficiently well-matched signal phases of multiphase analog triangular waveforms produced by a number of separate analog triangular waveform generators. While it is possible to digitally control a phase shift and frequency or time period of each of these analog triangular waveforms by the application of clock frequency locked digital control signals, maintaining accurate control of the amplitude and offset voltage of the multiphase triangular waveforms produced by such separate triangular waveform generators presents a challenge. The latter signal characteristics are typically determined by active and passive analog components and elements of the triangular waveform generator such as current generators, capacitors, resistors and switches. The values of the latter components inherently possess a certain amount of variation due to production spread or tolerances. While certain well-known integrated circuit design and layout techniques can be utilized to reduce these component variations between the separate analog triangular waveform generators these techniques are insufficient or impractical to make such separately generated multiphase triangular waveforms sufficiently accurate to reach optimal performance in multilevel Class D amplifiers.
This lacking accuracy or matching of the multiphase PWM signals degrades various important performance metrics of multilevel Class D amplifiers such as power efficiency, flying capacitor stability and general audio performance. Accordingly, there is a need in the art for a multiphase pulse width modulator producing multiphase pulse width modulated signals with improved accuracy and matching.